1. Field of the Invention
The present invention relates to an information recording medium that is particularly used for recording information and a reproducing apparatus for reading out information recorded in the information recording medium with making the information recording medium move relatively, particularly, relates to an information recording medium for recording and/or reproducing information optically and a reproducing apparatus thereof.
2. Description of the Related Art
Until now, there existed a system used for reading out information from an information recording medium while the information recording medium is made relatively move. In order to reproduce the system, such a method as optical, magnetic or capacitance is utilized. A system for recording and/or reproducing information by the optical method has been most popular in daily life. In the case of a read-only type information recording medium in disciform, which is reproduced by a light beam having a wavelength of 650 nm, for example, such a medium in disciform as a DVD video disc pre-recorded with picture image information, a DVD-ROM disc that is pre-recorded with a program or like, a DVD audio disc, or an SACD (Super Audio CD) disc that is pre-recorded with musical information is popularly known.
In the case of a recording and reproducing type information recording medium, there existed a DVD RAM disc utilizing a phase change effect, an ASMO (Advanced Storage Magneto-Optical) disc and an ID (intelligent image disc) utilizing a magneto-optical effect.
On the other hand, in order to increase recording density, such a study as shortening a wavelength of laser beam so as to realize emission of violaceous light has been continued. A second harmonic oscillating element or a semiconductor light emitting element of gallium nitride system compound, which was invented recently, emits light having a wavelength λ in the neighborhood of 350 nm to 450 nm. Consequently, they are possible to be an important light emitting element, which increases recording density drastically.
Further, a design of objective lens complying with such a wavelength has been advanced, Particularly, an objective lens having an NA (numerical aperture) utilized for a DVD disc, that is, an NA of exceeding 0.6 and more than 0.7 is being developed.
As mentioned above, a reproducing apparatus for information recording medium that is equipped with a light emitting element of which wavelength λ is reduced down to 350 nm to 450 nm and equipped with an objective lens of which an NA is more than 0.7 is being developed. By using these technologies, it can be expected that an optical disc system, which surpasses recording capacity of current DVD disc further more, will be developed.
Further, it is also desired that an information recording medium having higher recording density, which is designed on the basis of a violaceous laser beam and a higher NA, is developed.
On the other hand, a recent recording and reproducing type disc adopts a microscopic configuration, namely the land-groove system. With referring to FIGS. 41 and 42, an information recording medium designed for a higher NA recording and reproducing system is explained.
FIG. 41 is a cross sectional view of a conventional information recording medium adopting the microscopic configuration that is called the land-groove system according to the prior art.
FIG. 42 is an enlarged plan view of the information recording medium shown in FIG. 41 showing the horizontal configuration of the information recording medium according to the prior art.
As shown in FIG. 41, an information recording medium 100 is composed of a recording layer 120 and a light transmitting layer 110 that are sequentially laminated on a substrate 130. A microscopic pattern 131 is formed on the substrate 130. The recording layer 120 is formed directly on the surface of the microscopic pattern 131. The microscopic pattern 131 is composed of a plural of land portions “La” and “Lb” (hereinafter generically referred to as land portion “L”) and a plural of groove portions “Ga” to “Gc” (hereinafter generically referred to as groove portion “G”). Macroscopically, the configuration corresponds to that the microscopic pattern 131 is constituted by a continuous groove composed of the land portion “L” and another continuous groove composed of the groove portion “G”.
Further, as shown in FIG. 42, a record mark “M” is formed in both the grooves composed of the land portion “L” and the groove portion “G” respectively when recording.
With paying attention to the dimensions of the microscopic pattern 131, while a shortest distance between the groove portions “Ga” and “Gb” is assumed to be a pitch “P0” (another shortest distance between the land portions “La” and “Lb” is also the pitch “P0”), the microscopic pattern 131 is formed so as to satisfy a relation of P0>S0, wherein “S0” is a spot diameter of reproducing light beam.
Hereupon, the spot diameter “S0” is calculated by a wavelength λ of laser beam for reproducing and an NA of objective lens such as S0=λ/NA. In other words, the pitch “P0” is designed so as to satisfy a relation of P0>λ/NA.
In the case of the information recording medium 100, a light beam for recording (recording light) is irradiated on the light transmitting layer 110 and a record mark “M” is formed on both the land portion “L” and the groove portion “G” of the recording layer 120.
Further, a light beam for reproducing (reproducing light) is irradiated on the substrate 130 or the light transmitting layer 110 and reflected by the recording layer 120, and then the reflected reproducing light is picked up for reproducing.
Inventors of the present invention have actually manufactured an information recording medium 100 as an experiment, and experimentally recorded and reproduced the information recording medium 100. The inventors founded a problem such that a cross erase phenomenon was extremely noticeable. The cross erase phenomenon is a phenomenon such that information is recorded with being superimposed on a signal previously recorded in a groove portion “G”, for example, when recording the information in a land portion “L”. In other words, it is such a phenomenon that information previously recorded in a groove portion “G” is erased by recording another information in a land portion “L”.
Further, this phenomenon can also be noticeable in a reverse case, that is, the cross erase phenomenon is also recognized if previously recorded information in a land portion “L” is observed when recording information in a groove portion “G”. If such a cross erase phenomenon occurs, as mentioned above, information recorded in an adjacent groove is damaged. In case of an information system having larger capacity, an amount of lost information becomes excessively large. Consequently, affection to a user is enormous.
Consequently, it is considered for such an information recording medium 100 that information shall be recorded only in either land portion “L” or groove portion “G”. However, there is existed a problem such that recording capacity of an information recording medium will decrease and a merit of the information recording medium having a potential of recording in higher density will decline if such an information recording method is conducted.